Add a .one method to RecordCollections

It's a common use-case to want one and only one result from a query.
This adds a .one method to RecordCollections that is parallel to .all.

records.py

@@ -188,6 +188,30 @@ class RecordCollection(object):
     def as_dict(self, ordered=False):
         return self.all(as_dict=not(ordered), as_ordereddict=ordered)
 
+    def one(self, default=None, as_dict=False, as_ordereddict=False):
+        """Returns a single record for the RecordCollection, or `default`."""
+
+        # Try to get a record, or return default.
+        try:
+            record = next(self)
+        except StopIteration:
+            return default
+
+        # Ensure that we don't have more than one row.
+        try:
+            next(self)
+        except StopIteration:
+            pass
+        else:
+            raise ValueError('RecordCollection contains too many rows.')
+
+        # Cast and return.
+        if as_dict:
+            return record.as_dict()
+        elif as_ordereddict:
+            return record.as_dict(ordered=True)
+        else:
+            return record
 
 class Database(object):
     """A Database connection."""

tests/test_records.py

@@ -2,6 +2,8 @@ from collections import namedtuple
 
 import records
 
+from pytest import raises
+
 
 IdRecord = namedtuple('IdRecord', 'id')
 
@@ -49,6 +51,32 @@ class TestRecordCollection:
         assert len(rows) == 10
 
 
+    # all
+
+    def test_all_returns_a_list_of_records(self):
+        rows = records.RecordCollection(IdRecord(i) for i in range(3))
+        assert rows.all() == [IdRecord(0), IdRecord(1), IdRecord(2)]
+
+
+    # one
+
+    def test_one_returns_a_single_record(self):
+        rows = records.RecordCollection(IdRecord(i) for i in range(1))
+        assert rows.one() == IdRecord(0)
+
+    def test_one_defaults_to_None(self):
+        rows = records.RecordCollection(iter([]))
+        assert rows.one() is None
+
+    def test_one_default_is_overridable(self):
+        rows = records.RecordCollection(iter([]))
+        assert rows.one('Cheese') == 'Cheese'
+
+    def test_one_raises_when_more_than_one(self):
+        rows = records.RecordCollection(IdRecord(i) for i in range(3))
+        raises(ValueError, rows.one)
+
+
 class TestRecord:
 
     def test_record_dir(self):